Tahini comes in all sorts of shapes and sizes.  The very best is imported from Israel and can be found at speciality food stores.  I was instructed to simply buy the "blue one" because it is so delicious.  The shop owner concurred.  Seasoning the Tahini is an art form and all hinges on the proper balance of salt and lemon.  You will know when you get it just right because you instantly stick your finger back in the bowl for more.